This invention relates to a device for measuring individually the content of each type of substance in a suspension with at least two types of suspended substances provided the density of the substance in each type is approximately equal and the densities of the substances included in the individual types are known.
In preparation of paper, especially fine paper, a so-called stock is used, which consists of a mixture of fiber material and filler, usually kaolin, in a suspension. The stock is applied to the endless wire of a paper making machine, part of the particles of the stock remaining on the wire and forming a sheet while the remaining part of the particles passes through the wire together with the major portion of the liquid and forms the so-called white water. The term retention is used as a measure of the efficiency of the paper making machine. This term is the quotient between the particle material forming the sheet and the material supplied to the wire. Addition of chemicals will enable a control of the retention, which will make it possible, among other things, to give the sheet a high content of filler. In order to enable control of the retention measuring equipment is needed giving a measure of the retention, especially for the two particle types individually, as it may be difficult to obtain a sufficient retention of filler. In the control the so-called first pass retention is interesting, which is defined EQU 1-S.sub.BV /S.sub.IL
where
S.sub.IL =the concentration of studied particle type of the suspension applied to the paper making machine. PA1 S.sub.BV =the corresponding concentration of the particles of the white water. PA1 S.sub.M is the content of the suspended substance of the second substance type, PA1 U.sub.1 is the signal received from the density meter by the calculating unit, PA1 U.sub.2 is the signal received from the apparatus for measurement of particle concentration by the calculating unit, and PA1 k.sub.a, k.sub.b, k.sub.c and k.sub.d are constants of proportionality.
Now the acute need is to measure the concentration of filler of stock and white water independently of the presence of fiber material. A procedure often used with retention control comprises measurement of turbidity and density each separately. In measurement of the turbidity which is most sensitive to clay a device is used in which the damping of light passing through a particle suspension is determined. The essential disadvantage of this procedure is the dependence of a signal sensitivity on the particle size. Difficulties will arise in establishing whether the particle concentration or the particle size or both have changed when the resulting measuring signal is changed. Certainly the density measurement is sensitive to the change in concentration of the investigated suspension but measurement of clay is disturbed by the presence of fibers. For physical reasons there is no great changes of success when combining turbidity and density measurement.